1. Field of the Invention
The present invention relates to a method of producing a phase shift mask of attenuated type, particularly to a method of forming an optical proximity correction for weakening side lobe lights in the phase-shift mask of attenuated type.
2. Discussion of Background
In recent years, microminiaturization of a circuit pattern is increasingly pursued in accordance with large integration of a semiconductor integrated circuit. Specifically, photolithography technology plays the most important role in forming such a microminiature circuit pattern. A main subject in the photolithography technology resides in how accurately a microminiature resist pattern is formed. For this, a major problem to be solved is to improve resolution into a pattern and a depth of focus. There have been proposed many technologies for improving this resolution and this depth of focus. In these technologies, a method of improving a photo mask has been paid special attention to because the method is simple and practical. Among various types of technologies concerning the improved photo masks, for example, a phase shift mask of attenuated type reported by N. Yoshioka et al.,: IEDM (International Electron Devices Meeting) 93, P. 653 (1993) has been especially paid attention to and has been practically used in these days because the mask has many advantages not provided in the other improved photo masks, such as applicability of the conventional process using a positive resist and small restriction against pattern layout.
This phase shift mask of attenuated type can improve contrast of edge portions of an optical image and, particularly, is effective for improving resolution and a depth of focus of a pattern of isolated holes by providing a phase shift film of attenuated type having a transmittance of about 2% through 20% with respect to a wavelength of exposure and a phase shift of 180.degree. with respect to an aperture, instead of an ordinary shading film of photo mask.
However, in such a phase shift mask of attenuated type, there remains problems with pattern layout. FIG. 15 is a plan view for showing a conventional phase shift mask of attenuated type. In the Figure, a numerical reference 1 designates a phase shift film of attenuated type; a numerical reference 2 designates a pattern with concentrated apertures composed of rectangular apertures 2a through 2i formed by opening the phase shift film of attenuated type; and numerical references 3 and 4 respectively designate patterns with an isolated aperture composed of a rectangular aperture.
The phase shift mask of attenuated type includes the patterns with an isolated aperture 3 and 4 which do not have any aperture around the periphery thereof and the pattern with concentrated apertures 2 having a plurality of apertures which are arranged in the directions of X and Y in an array-like shape and of which opening width W.sub.1 and a width between adjacent openings W.sub.2 in the X direction (or the Y direction) are in a ratio of about 1:1 through 1:2. In a case that a resist pattern is formed using such mask, when a luminous exposure is optimized using the pattern with an isolated aperture 3 or 4, a pattern defect called as a dimple is generated in portions designated by A through D in the pattern with concentrated apertures 2 shown in FIG. 15, wherein such dimple was not originally anticipated in making design data.
In FIGS. 16(a) through 16(d), a schematical view for explaining a reason for generating such defect of resist pattern is shown. FIG. 16(a) shows a cross-sectional view of the conventional phase shift mask of attenuated type taken along a line I--I of FIG. 15; FIG. 16(b) shows amplitude in a wafer face of exposure light transmitted through the photo mask; FIG. 16(c) shows intensity of the exposure light on the wafer face; and FIG. 16(d) shows a cross-sectional view of a resist pattern formed by the exposure light.
For example, the exposure lights transmitted through apertures 2i, 2e and 2a of the pattern with concentrated apertures 2 and the exposure lights transmitted through the phase shift mask of attenuated type 1 overlap partially each other in an exposed region by the exposure lights transmitted through the phase shift film of attenuated type 1 as shown in FIG. 16(b). Because of these overlaps of the exposure lights, a side lobe light 302 having large intensity was generated in the exposed region of the phase shift film of attenuated type 1 as shown in FIG. 16(c). As a result, as shown in FIG. 16(d), the resist in other than a region which should have inherently been exposed by only a main peak light 301 transmitted through the aperture 2i, 2e or 2a was exposed by the side lobe light, whereby a pattern defect 402 with a loss of the resist, which has not been planed in original design data, occurred in the resist pattern 401.
In the above description, only the overlap of exposure lights from adjacent apertures in the line I--I is considered. However, this overlap practically was two-dimensional, whereby more exposure lights were overlapped and the intensity of side lobe light at the portions A through D, which were symmetric portions with respect to the apertures, became the strongest.
FIG. 17 is a picture in a plan view taken by a scanning electron microscope (SEM) when a resist pattern is actually formed on a wafer using the conventional phase shift mask of attenuated type. In this Figure, a pattern defect 402 was generated at a periphery of a resist aperture.
In the U.S. Pat. Nos. 5,487,963 and 5,591,550, there is proposed phase shift masks of attenuated type which restrict occurrence of pattern defects by forming an optical proximity correction formed by a shading film or apertures at portions corresponding to portions, at which side lobe lights are generated, in order to cope with pattern defects that occur at a time of using such conventional phase shift mask of attenuated type. FIGS. 18 and 19 respectively are a plan view of the conventional phase shift mask of attenuated type provided with a shading film as an optical proximity correction in the phase shift mask of attenuated type shown in FIG. 15 and a cross-sectional view taken along a line II--II of this plan view. FIGS. 20 and 21 respectively are a plan view of the conventional phase shift mask of attenuated type provided with apertures as an optical proximity correction and a cross-sectional view taken along a line III--III of this plan view.
These methods were to relax overlaps of exposure lights and reduce side lobe lights by disposing an optical proximity correction 5 made of, for example, a shading film 5a on the corresponding portions to those with the side lobe lights generated in the phase shift mask of attenuated type 1 or by providing an optical proximity correction 6 formed by, for example, an aperture 6a of the phase shift mask of attenuated type 1.
In the next, a method of producing a phase shift mask of attenuated type having such-optical proximity correction will be described.
At first, a method of producing a phase shift mask of attenuated type having an optical proximity correction made of a shading film will be described.
FIGS. 22(a) through 22(d) and FIGS. 23(e) through 23(h) respectively are cross-sectional views of the mask taken along the line II--II of FIG. 18 for showing processes of producing the phase shift mask of attenuated type shown in FIG. 18.
In FIG. 22(a), a phase shift film of attenuated type is formed on a light transmittible substrate. In the next, a resist for electron beam 8 is formed on the phase shift film of attenuated type 1. As shown in FIG. 22.(c), an electron beam is irradiated upon the resist for electron beam 8 depending on pattern data of an electron beam lithography system for forming the pattern of apertures shown in FIG. 18; thereafter the resist is developed to thereby obtain a desirable resist pattern for electron beam. As shown in FIG. 22(d), the phase shift film of attenuated type 1 is etched using this resist pattern 9 as a mask and thereafter the unnecessary resist is removed in order to form a pattern with concentrated apertures formed by the apertures 2a through 2i in the phase shift mask of attenuated type 1. In this, the photo mask completed at this step is the conventional phase shift mask of attenuated type without the optical proximity correction described in the above.
In the next, in FIG. 23(e), a shading film 10 is formed on the phase shift film of attenuated type 1 in which the pattern with concentrated apertures is formed and the light transmittible substrate 7 which is exposed. As shown in FIG. 23(f), a resist for electron beam 8 is formed again on the shading film 10. As shown in FIG. 23(g), an electron beam is irradiated on the resist for electron beam 8 based on the data of pattern of the electron beam lithography system for writing the optical proximity correction, and thereafter the resist is developed to thereby obtain a resist pattern for electron beam 11. In this, the data of pattern for writing such optical proximity correction are obtained by conducting various transfer tests, various optical simulations or the like, for example, a resist pattern is practically formed using the conventional phase shift mask of attenuated type without an optical proximity correction in order to obtain data concerning originating portions of pattern defects, conditions of a transfer and so on, which are generated with respect to different pattern sizes and pattern alignment. As shown in FIG. 23(h), the shading film 10 is subjected to an etching using this resist pattern 11 as a mask and thereafter the unnecessary resist is removed, whereby the phase shift mask of attenuated type having the optical proximity correction 5 composed of the shading films 5a and 5c is completed.
In the next, a method of producing a phase shift mask of attenuated type having an optical proximity correction formed by apertures will be described. The method of producing is processed as shown in the FIGS. 22(a) through FIG. 22(d) and further the FIGS. 24(e) through FIG. 24(g), which are cross-sectional views of the mask for showing the manufacturing process.
As shown in FIG. 24(e), a resist for electron beam 8 is again formed on the phase shift mask of attenuated type 1. As shown in FIG. 24(f), an electron beam is irradiated to the resist for electron beam 8 based on data of pattern of an electron beam lithography system for writing an optical proximity correction, and thereafter the resist is developed, to thereby obtain a resist pattern 12. In this, the data of pattern of the optical proximity correction are obtained by conducing various transfer tests, various optical simulations or the like as in the above case of forming the optical proximity correction composed of the above shading portions. As shown in FIG. 24(g), a part of the phase shift film of attenuated type 1 is etched using this resist pattern 12 as a mask and the unnecessary resist is removed, whereby the phase shift mask of attenuated type having the optical proximity correction 6 composed of apertures 6a and 6c is completed.
However, the method of producing the phase shift mask of attenuated type having the conventional optical proximity correction had problems that the method was not practically applicable such that the cost of production was high and throughput was very low, because in order to optimize the optical proximity correction, which were different at each pattern size or at each pattern arrangement, the phase shift mask constructed as described in the above required, for example, various transfer tests, various optical simulations or the like by practically patterning the resist using the phase shift mask of attenuated type without the optical proximity correction to obtain data about a location of the optical proximity correction and data about conditions of the transfer, making of data of the pattern for writing the optical proximity correction of the electron beam lithography system based on thus obtained data, and a very large amount of data for writing for electron beam.